In order to increase the information storage density or track density, an optical disk unit that records/reproduces information by emitting a light beam at a disk storage is required to ensure accurate track-following of the beam to target disk-tracks or accurate focusing (focus control) in response to disk surface runout.
Generally, the accuracy of tracking control is improved by increasing the loop gain of a control system to provide the control loop with a higher response frequency band. However, there is a limit to characteristics of a drive mechanical system for driving a moving member, and it is often the case that enough accuracy cannot be secured.
One conceivable approach to resolve this problem involves the use of regularity in runout on the surface or recording track of a disk storage to reduce the error. That is, the rotation of a disk causes positional shifts or up-and-down movements on a disk surface, and the main elements of the shifts are substantially synchronized with the rotation of a disk. Consequently, by taking advantage of the periodicity of shifts in information recorded/recording position (objective member) on a disk, the tracking performance of the moving member (a light beam spot) for the objective member can be improved with the use of a position error signal generated by the last or more previous rotation.
There is found an example of such technique in Japanese Patent publication No. SHO60-57085 (first prior art). The positioning control device of the first prior art is provided with a signal delay unit for integrating/accumulating position error signals in synchronism with a rotation period for position shifts or fluctuations having constant periodicity. The position error signal of each moment is added/input in the signal delay unit, and the sum of the output of the signal delay unit and the position error signal is input in a drive unit for a moving member.
According to the first prior art, when the periodic position shift occurs n times, a relative position error Xe is given by:Xe=Xi/{1+G(s)}nin which G(s) is a transfer function of the drive unit, and Xi is the periodic position shift of an objective member. This indicates that the output of the delay unit approaches Xi/G(s).
Namely, in a frequency domain where the absolute value of 1+G(s) is bigger than 1, the relative position error approaches 0, and a signal for making the moving member react to the periodic position shift is almost given by the output of the signal delay unit. Consequently, the tracking performance can be vastly improved without too much increase in the gain (the absolute value of G(s)) of a drive system or response frequency band (frequencies where the gain of G(s) is 1).
PROBLEMS THAT THE INVENTION IS TO SOLVE
According to the first prior art, however, there is a problem in that the error is reduced only in the frequency domain where the absolute value of 1+G(s) is sufficiently bigger than 1 and, on the contrary, in the case where 1+G(s) is smaller than 1, the error expands (diverges) rather than contracts.
For example, when the drive unit has a second order phase-delay characteristic, if the phase is delayed by nearly 180 degrees at the frequency where the gain (absolute value) of G(s) is close to 1, 1+G(s) shows a value considerably smaller than 1.
In an ordinary control system, 1/(1+G(s)) represents how much existent displacement or offset can be reduced to a little position error by control, namely, error reduction ratio, and the value of 1+G(s) smaller than 1 is observed as a phenomenon in which the position error becomes bigger than its original value in the vicinity of the cut-off frequency (frequency where the gain of G(s) is 1=0 db) of the control loop.
With this construction of the first prior art, the error signal gets bigger and bigger or diverges in a frequency band where 1+G(s) is smaller than 1.
Accordingly, the application of the first prior art demands some kind of measures to avoid that prospect. One approach involves the use of a low-pass filter that attenuates signals input to the signal delay unit in the vicinity of the cut-off frequency of the control loop before inputting the sum of the output of the signal delay unit and the position error signal into the signal delay unit. In other words, according to the first prior art, the displacement of an information recorded/recording position (objective member) on a disk, which synchronizes with the rotation of the disk, can be reduced only in a frequency range which satisfies the condition that the gain of the drive unit is adequate and the value of 1+G(s) is sufficiently bigger than 1, and in a frequency range where 1+G(s) approximates to 1 or is smaller than 1 (a range of frequencies higher than the vicinity of the cut-off frequency of the control loop), no error reducing effect can be expected.
It is therefore an object of the present invention to provide a positioning control device and a positioning control method for reducing the relative position error in tracking by using the periodicity of position shift or fluctuation without unnecessarily increasing the gain and frequency band being the transfer characteristics of the control system.